Apple TV+ and Apple TV Hardware 1st Gen

You replace it with a Apple TV HD or Apple TV 4K model. Apple TV's prior to Apple TV 3's cannot access Apple TV +. If you are unsure of the generation of your Apple TV please click https://support.apple.com/en-us/HT200008

If you really had a 1st gen (big aluminum box) then doesn't have access to any 3rd party content, no longer has access to the iTunes store or syncing and is EOL.

If you have a small black box then it's a 2nd or 3rd gen. You would need at least a 3rd gen to have access to the TV app, which then lets you access Apple TV +

A 3rd gen would need to be updated to at least firmware version 7.3 while the HD and 4K models would require TVOS 12.3 or later.

Identify your Apple TV model - Apple Support

How to update your Apple TV - Apple Support